Material ablation by pulsed light sources has been studied since the invention of the laser. Reports in 1982 of polymers having been etched by ultraviolet (UV) excimer laser radiation stimulated widespread investigations of the process for micromachining. Since then, scientific and industrial research in this field has proliferated—mostly spurred by the remarkably small features that can be drilled, milled, and replicated through the use of lasers.
Ultrafast lasers generate intense laser pulses with durations from roughly 10−11 seconds (10 picoseconds) to 10−14 seconds (10 femtoseconds). Short pulse lasers generate intense laser pulses with durations from roughly 10−10 seconds (100 picoseconds) to 10−11 seconds (10 picoseconds). A wide variety of potential applications for ultrafast lasers in medicine, chemistry, and communications are being developed and implemented. These lasers are also a useful tool for milling or drilling holes in a wide range of materials. Hole sizes as small as a few microns, even sub-microns, can readily be drilled. High aspect ratio holes can be drilled in hard materials, such as cooling channels in turbine blades, nozzles in ink-jet printers, or via holes in printed circuit boards.
The ability to drill holes as small as microns in diameter is a basic requirement in many high-tech manufacturing industries. The combination of high resolution, accuracy, speed, and flexibility has allowed laser processing to gain acceptance in many industries, including the manufacture of integrated circuits, hard disks, printing devices, displays, interconnects, and telecommunication devices.
There exist multiple methods for laser machining; however, when fine features are to be drilled, tolerances are smaller for the finished product in laser micromachining. In this case, the process used must provide consistent, predictable, and repeatable results to satisfy the end application. Computer control via algorithms and software in laser micromachining provides the opportunity for fine control of hole geometry and the consistency required for a profitable, mass-production manufacturing facility. This opportunity should not be squandered, as many problems continue to exist related to micromachining.
One problem that persists in the field relates to avoiding manufacturing off-specification products with micromachining. This problem is persistent because, in micromachining, the tolerance for error is low and consistency is critical from product to product. For example, inkjet nozzle holes must be manufactured consistently to provide equal ink ejection from each hole when used. When a process is not consistent or repeatable, the manufacturing line produces off-specification products that result in wasted time and energy, mandatory rework, and reduced throughput. This in turn reduces profitability of a manufacturing facility. What is needed is a way to avoid manufacturing off-specification products with micromachining. Another persistent problem related to micromachining involves production of consistent, repeatable results in milling. As noted above, consistency and repeatability are important factors in producing technically acceptable, high quality micro-machined products. However, current methods of milling are not designed to ensure that the required hole geometry is consistent from item to item in the manufacturing line. What is needed is a way to produce consistent, repeatable results in milling.
A further persistent problem relating to micromachining involves providing guidelines for creating tool path geometry; in recent history, milling techniques that produce predictable and repeatable hole geometries have proven difficult to achieve. Trial and error methods have been used to manufacture desired hole geometries: parameters are iteratively changed to reach the desired shape. A typical procedure is to step through the desired tool path radius linearly over time; however, this technique introduces uneven pitches in the spiral path, which causes variations in the radial overlap. The uneven ablation that results is undesirable. An algorithmic approach proves mildly successful, in that a desired shape is produced using a constant angular velocity and tool pitch. However, this process does not compensate for the spacing of exposure steps generated near the center of the hole as shown in FIG. 1. What is needed is a way to provide guidelines for creating tool path geometry.
A still further persistent problem relating to micromachining involves providing a laser drilling system tool path allowing for constant material removal. Current requirements for milling require total material ablation across the workpiece target area. Past techniques include such methods as excimer laser ablation and a constant angular velocity approach, shown in FIG. 1. However, these techniques do not provide the flat surface required by customer specifications. What is needed is a way to provide a laser drilling system tool path allowing for constant material removal. A still further persistent problem relating to micromachining involves maintaining constant exposure of a laser source on a workpiece when the tool path is changing. In a constant pulse laser system, the laser is pulsed at a fixed repetition rate; therefore, the uniform ablation is translated into a required constant propagation speed of the laser strike point onto the workpiece. When using a semi-circular motion, such as spiraling, the linear speed of the strike point should be constant throughout the laser milling process to maintain constant ablation. What is needed is a way to maintain constant exposure of a laser source on a workpiece when the tool path is changing.